A common issue for gas turbine power plants is the treatment of exhaust gases to comply with laws restricting pollutants present in the gases emitted into the ambient atmosphere. A common, commercially available gas turbine power plant is a simple cycle power plant. Simple cycle power plants are frequently utilized as peaking power plants which generate electricity typically during a high demand, known as peak demand. The critical features to meeting the peak demand are fast start/shut down and cyclic capabilities. In a simple cycle turbine ambient air is compressed and at high pressure mixed with fuel generating thermal energy in a turbine combustion chamber. The high temperature gas is expanded in the turbine and its energy is extracted and converted into mechanical work. Finally, the high temperature and low pressure exhaust gas leaves the gas turbine.
As a result of the combustion process, numerous species present in the fossil fuel and combustion air are oxidized. For instance, high combustion temperature accelerates oxidation of the atmospheric nitrogen that is present in the inlet combustion air and converts to oxides organically bound nitrogen-based species present in the fossil fuels. As a result, exhaust gas produced by gas turbines contains nitrogen oxides (NOx) which consists primarily of nitric oxide (NO) and nitrogen dioxide (NO2). After release, in the ambient atmosphere, NO is oxidized to secondary pollutants as NO2 and organic nitrates which in turn trigger reactions producing ozone and other radicals. NO2 is a toxic yellow-brown gas that is a major component of photochemical smog in urban areas, which contributes to formation of acid rain and is a precursor to low-level ozone formation. A detailed discussion of the NO role in photochemistry of the troposphere and stratosphere is given by V. I. Parvulescu at al., “Catalytic Removal of NO,” Catalysis Today, 46, 1998, 233-316.
In addition to formation of NOx, the combustion process generates numerous other oxides, some of which are produced as a result of partial oxidation of fuels in the combustion zones with reduced availability of oxygen (O2). An example of these oxides is carbon monoxide (CO) that is a very stable molecule and highly toxic to humans.
Typical concentrations of NOx and CO in the raw turbine exhaust gas are in the range of about 10 to 100 ppmvdc (parts per million by volume, on a dry or water-free basis, corrected to 15 percent oxygen) depending on the type and mode of the simple cycle turbine operation. However, the US government agencies and local environmental authorities have established emission limits for NOx that are typically in the range from below 2 to 5 ppmvdc. It is, therefore, necessary to reduce the NOx concentration in the exhaust gas before it can be released to the atmosphere. The limits established for CO emissions may require over 90 percent reduction of raw emission levels. Typically the air permit for plant operation states the allowable emission levels for pollutants present in the exhaust gas. To comply with these regulations the raw emissions of NOx, CO, unburned hydrocarbons (UHC), volatile organic carbons (VOC) and other regulated pollutants must be reduced and maintained below the permitted values.
Whereas many techniques have been developed for reduction of emissions by modifying turbine combustion characteristics, only post-combustion exhaust gas cleaning technologies are capable of reducing NOx, CO, UHC and VOC concentrations below 5 ppmvdc. To comply with environmental standards simple cycle power plants are equipped with catalytic systems to reduce concentration of CO, UHC, VOC, NOx and other hazardous components present in the exhaust gas. The catalytic treatment of the exhaust flue gas is generally considered as a Best Available Control Technology (BACT) that represents the most stringent emissions control process to be technologically feasible and cost effective.
Such catalytic treatment of pollutants requires systems with substantial footprints to accommodate the emissions control catalysts, such as seen in FIG. 1. Moreover, in order to efficiently treat the exhaust gas for different emissions, the temperature of the exhaust gas must be controlled for optimal catalyst performance and capital and operating cost effectiveness. The existing designs utilize ambient air (so called tempering air) injection systems to reduce the exhaust gas temperature. The exhaust gas is cooled down mainly to reduce operating temperature of the emission control catalysts. However, different catalysts developed to control CO, UHC, VOC and NOx operate efficiently at different operating temperatures. Consequently, maintaining one range of exhaust gas temperature for all catalysts by pre-cooling exhaust gas leaving the turbine results in inefficiency of catalytic processes and high exhaust gas pressure drop. This also increases the footprint required for the exhaust system.
Accordingly, there is a need to develop a compact exhaust system and efficient tempering air arrangement for simple cycle turbines, and other similar combustion systems, minimizing capital and operating costs and maximizing efficiency of the emissions control catalysts.